1. Field of the Invention
This invention relates to chain drive and power transmission systems and more particularly, to a new and improved sprocket for driving chains, and a method for producing the sprocket, which sprocket is characterized by a tooth profile having clearance and loading curves tailored to a selected chain barrel diameter, a selected pressure angle and flared sides with a selected topping curve radius. The sprocket is designed to receive the barrels or drums of a chain and to accept the driving load in a fashion that prolongs service life by minimizing contact stresses while at the same time maintaining acceptable chain release characteristics.
An active chain and power transmission system is intended to carry a load, producing full chain tension along the active chain strand, and to be driven about a revolving sprocket where the load decreases in steps to that of the return or catenary strand. A cooperating sprocket then produces a stepped-up tension on the chain to produce a characteristic loading and unloading of the working chain. It is the smooth acceptance and release of the chain from the sprocket and the incremental change of load on the engaged sprocket teeth which are of paramount interest in sprocket design.
Chain barrel to sprocket tooth interface loads decrease incrementally around the drive sprocket from a maximum load on the sprocket tooth which initially engages the chain to a minimum on the sprocket tooth located at the balance point between the driving strand and slack strand tension. The major driving load typically occurs in a time interval of about 100 milliseconds when this type of power transmission system operates at normal speeds, producing a load similar to a heavy hammer blow against the face of the sprocket tooth which first engages the chain in a given interval of time. These repeated shocks result in rapid wear of the sprocket teeth.
The shock load is applied at the interface of the chain barrel and straight side of the sprocket tooth in a conventional trapezoidal sprocket system. Accordingly, the barrel-tooth contact can be considered for design calculation purposes to be a cylinder working on a plane. Contact stresses on such sprockets are quite high, and frequently result in permanent deformation of the sprocket teeth and shortened service life. Such stresses are also increased by failure of the chain barrels and sprocket teeth to make uniform contact during engagement.
Great care is generally exercised by all users in the selection of engineering and mill class chains for power transmission systems, and chain load ratings published by chain and sprocket manufacturers are usually conservatively stated in order to insure long chain life. Furthermore, several construction options, all of which are designed to improve service life, are generally offered to insure that the user selects a chain which will be used in a service which is well within the chain load capacity. Such options generally include a choice of hardened pins and bushings, and either carbon steel or a special alloy steel can be specified as a desired material of construction. These offerings generally permit the selection of a chain that will afford the user extended service life and an excellent return on the chain investment.
In contrast, sprockets have received much less attention than the chain systems with regard to design and care in specification. It is generally accepted in the industry that sprockets will wear to a certain degree after a relatively short service life and will in the normal course of events require replacement or extensive reworking prior to being placed back into service. The sprockets most frequently develop a cavity, and wear "hooked" due to attrition between the chain barrel or drum and the sprocket teeth, generally because of high magnitude shock loading as the srocket teeth initially engage the active chain strand. This relatively rapid wear results in considerable down-time required to replace or modify the sprockets, and this, in combination with a steadily increasing labor cost factor, make such lost time a costly hindrance.
The typical mill chain sprocket currently in use is constructed by means of either a sand casting or it is flame cut from carbon steel. Such sprockets are intended for use with a large variety of chains having identical pitch but not necessarily the same barrel or roller diameter. An advantage is realized in this type of construction because the sprockets so constructed are relatively inexpensive, and the stocking distributor and user are therefore able to stock a reduced inventory; accordingly, the replacement cost of such sprockets is relatively low and the availability is high.